A. Field of the Invention
This invention relates to the field of line of sight stabilization for optical systems. More particularly, the invention relates to a stabilization system in which complementary plano-concave and plano-convex lens elements are moved relative to one another to correct for small amplitude accelerations, thereby causing light passing though the lens elements to refract and maintain alignment with the line of sight of the optical system.
B. Description of Related Art
Line of sight stabilization involves the maintenance of the line of sight (or optical axis of an optical system) in a steady orientation during periods in which the optical system is subject to disturbances, such as random, small amplitude jitter or accelerations. Systems for performing line of sight stabilization can take many forms. In one widely used form, the stabilized optical instrument is mounted on a platform, and the platform itself is stabilized using gyroscopes, internal rate sensors, or linear accelerometers. One implementation of this technique is disclosed in the Algrain patent, U.S. Pat. No. 5,124,938, assigned to the assignee of the present invention. In the Algrain patent, a gyroless stabilization system is provided in which a camera or other sensor is mounted to a platform in a vehicle. The line of sight of the sensor is stabilized about the center of rotation of the vehicle by moving gimbals to counteract accelerations about the center of rotation.
Another technique for image stabilization is to provide a complex arrangement of lenses and mirrors, and electronic circuitry for positioning the mirrors to stabilize the image. Several patents discussing this technique are U.S. Pat. Nos. 4,417,788 and 3,468,596, both issued to Alvarez.
Another technique for stabilizing the line of sight of a camera or other sensor takes advantage of the so-called "wedge effect". As shown in FIG. 1A, when a plano-concave lens 10 and a plano-convex lens 12 are placed in close proximity to one another such that the planar surfaces of the lens elements 10 and 12 are parallel, light rays passing through the lenses are not refracted from their original path. The illustration of FIG. 1A shows light passing from left to right, but the effect is the same for light passing through the lenses from right to left. Referring to FIG. 1B, if one of the lenses is rotated relative to the other, here, the plano-convex lens is rotated relative to the plano-concave lens, the parallel surfaces of the lens elements are no longer parallel and light is refracted through an angle .alpha.. The angle formed by the non-parallel sides of the lens elements is known as the wedge angle W. The angle of refraction .alpha. is related to the wedge angle W by the relationship: EQU .alpha.=2W
In the above discussion of FIG. 1B, the plano-concave lens element 10 is said to be "fixed" relative to the plano-convex lens element 12, because, in the example and in use, the plano-concave element 10 remains stationary while the plano-convex lens 12 is rotated in a spherical manner relative to the plano-concave lens 10. In the inertial frame of reference, the plano-convex lens element is fixed. The same refraction angle .alpha. and relationship between .alpha. and W occurs when the positions of the plano-concave lens 10 and plano-convex lens 12 are interchanged, or when the stabilized optical system emits photons rather than collects photons, as would be the case for a stabilized range finder or designator.
The wedge effect illustrated in FIGS. 1A and 1B has been used in prior art stabilization systems. FIG. 2 illustrates the system shown in U.S. Pat. No. 4,911,541 in a greatly simplified form. The outer plano-concave element 10 is fixed relative to the housing (not shown) of the optical system to be stabilized. The line LOS represents the line of sight of the optical system. The plano-convex element 12 is mounted to a set of gimbals 14, 16. The plano-convex 12 element is inertially stabilized by movement of the gimbals 14, 16. The gimbals 14, 16 rotate the plano-convex lens 12 relative to the plano-concave lens 14 when jitter and other small amplitude accelerations are detected. The wedge angle created by the rotation of the movable lens 12 relative to the fixed lens 10 refracts light passing though the lenses. By controlling the amount of movement of the gimbals, the refraction angle is also controlled. Ideally, the refraction angle is controlled such that the line of sight of the optical system is stabilized. Other patents disclosing the technique of FIG. 2, or a variant thereof, are U.S. Pat. Nos. 5,122,908 and 4,013,339. Other patents relating to image stabilization systems are U.S. Pat. Nos. 5,148,313, and 4,013,339.
A significant inherent limitation of the prior art system of FIG. 2 is that the gimbals 14, 16 and lenses 10, 12 are incorporated, along with the camera or other sensor, into an integrated and unitary sensor and stabilization structure. In particular, the gimbal and lens design of FIG. 2 is not readily susceptible to installation onto existing optical systems that do not have line of sight stabilization systems already built in. The present invention, however, permits a line of sight stabilization system to be fitted onto preexisting optical systems. Heretofore, such a result was highly impractical or impossible.
The application of line of sight stabilization capability onto preexisting optical system has tangible benefits. Frequently, cameras and other optical devices do not have line of sight stabilization compatibility as part of the original equipment specifications. This is often the case with military sighting and tracking systems. By virtue of the installation of line of sight stabilization capability on the existing optical system, the performance of the optical system is improved, because jitter and other unwanted small amplitude oscillations are removed.
Accordingly, it is an object of the invention to provide a line of sight stabilization system which is capable of installation onto a preexisting optical system.
Another object of the invention is to provide a line of sight stabilization system in which the line of sight stabilization system is compact in size.
An additional object of the invention is to provide a robust and rugged line of sight stabilization system suitable for use in conjunction with military optical systems, such as sighting and viewing devices, designating systems, rangefinding systems, and reconnaissance systems, and particularly optical systems used in ground combat.